Bottom Line:
The results show that in vivo these breast cancer models become dependent upon co-activation of FGFR and ErbB receptors for PI3K pathway activity.The work presented here shows that in the breast cancer models examined, the combination of dovitinib + NVP-BEZ235 or dovitinib + AEE788 results in strong inhibition of tumor growth and a block in metastatic spread.The resultant decrease in mitosis and increase in apoptosis was consistently stronger in the dovitinib + AEE788 treatment-group, suggesting that targeting ErbB receptors has broader downstream effects compared to targeting only PI3K/mTOR.

Introduction: Targeting receptor tyrosine kinases (RTKs) with kinase inhibitors is a clinically validated anti-cancer approach. However, blocking one signaling pathway is often not sufficient to cause tumor regression and the effectiveness of individual inhibitors is often short-lived. As alterations in fibroblast growth factor receptor (FGFR) activity have been implicated in breast cancer, we examined in breast cancer models with autocrine FGFR activity the impact of targeting FGFRs in vivo with a selective kinase inhibitor in combination with an inhibitor of PI3K/mTOR or with a pan-ErbB inhibitor.

Methods: Using 4T1 or 67NR models of basal-like breast cancer, tumor growth was measured in mice treated with an FGFR inhibitor (dovitinib/TKI258), a PI3K/mTOR inhibitor (NVP-BEZ235) or a pan-ErbB inhibitor (AEE788) individually or in combination. To uncover mechanisms underlying inhibitor action, signaling pathway activity was examined in tumor lysates and transcriptome analysis carried out to identify pathways upregulated by FGFR inhibition. Anti-phosphotyrosine receptor antibody arrays (P-Tyr RTK) were also used to screen 4T1 tumors.

Results: The combination of dovitinib + NVP-BEZ235 causes tumor stasis and strong down-regulation of the FRS2/Erk and PI3K/Akt/mTOR signaling pathways. P-Tyr RTK arrays identified high levels of P-EGFR and P-ErbB2 in 4T1 tumors. Testing AEE788 in the tumor models revealed that the combination of dovitinib + AEE788 resulted in blockade of the PI3K/Akt/mTOR pathway, prolonged tumor stasis and in the 4T1 model, a significant decrease in lung metastasis. The results show that in vivo these breast cancer models become dependent upon co-activation of FGFR and ErbB receptors for PI3K pathway activity.

Conclusions: The work presented here shows that in the breast cancer models examined, the combination of dovitinib + NVP-BEZ235 or dovitinib + AEE788 results in strong inhibition of tumor growth and a block in metastatic spread. Only these combinations strongly down-regulate the FGFR/FRS2/Erk and PI3K/Akt/mTOR signaling pathways. The resultant decrease in mitosis and increase in apoptosis was consistently stronger in the dovitinib + AEE788 treatment-group, suggesting that targeting ErbB receptors has broader downstream effects compared to targeting only PI3K/mTOR. Considering that sub-classes of human breast tumors co-express ErbB receptors and FGFRs, these results have implications for targeted therapy.

Figure 6: Analysis of 4T1 tumors from mice treated with AEE788 + dovitinib. (A) Frozen sections from 4T1 tumors harvested after 14 days of treatment as described in Figure 5A were stained for proliferation (phospho-histone H3; blue), apoptosis (cleaved caspase 3; red) and endothelial cells (CD31; green). Scale bars, 100 μm. (B) Phosphorylated histone-3 (PH3)-positive cells were manually counted and expressed as the average number of cells per field. Proportion of tumor area expressing cleaved Caspase-3 and CD31 immunoreactivity were quantified using ImageJ. N = 3 to 4 tumors for each group with five fields analyzed per tumor (15 to 20 images from each treatment). Bars represent mean ± SD in each treatment group. (C) 2.5 × 105 4T1 cells were injected into tail veins and indicated treatments were started 7 days later for a total of 11 days. Lungs were harvested and metastases were quantified. N = 5, representative of two separate experiments. *P < 0.05; **P < 0.01 (Mann-Whitney U-test).

Mentions:
The 4T1 tumors were collected at the endpoint and examined for proliferation, apoptosis and vessel density (Figure 6A). Quantification of P-Histone H3, cleaved Caspase-3 and CD31 revealed a significant decrease in cell proliferation and an increase in cell death, which was most prominent and significant in the dovitinib + AEE788-treated group (Figure 6B). The tumor vasculature area and morphology were significantly altered only after combination treatment, primarily as a result of the action of dovitinib as described earlier (Figure 2C). In the experimental metastasis model, treatment with AEE788 alone did not significantly lower the number of lung metastases, while the combination of dovitinib + AEE788 caused a highly significant decrease. These results support the hypothesis that combination therapy effectively inhibits primary tumor outgrowth by impairing proliferation and cell survival; while lung metastases are also very sensitive to blockade of both receptors.

Figure 6: Analysis of 4T1 tumors from mice treated with AEE788 + dovitinib. (A) Frozen sections from 4T1 tumors harvested after 14 days of treatment as described in Figure 5A were stained for proliferation (phospho-histone H3; blue), apoptosis (cleaved caspase 3; red) and endothelial cells (CD31; green). Scale bars, 100 μm. (B) Phosphorylated histone-3 (PH3)-positive cells were manually counted and expressed as the average number of cells per field. Proportion of tumor area expressing cleaved Caspase-3 and CD31 immunoreactivity were quantified using ImageJ. N = 3 to 4 tumors for each group with five fields analyzed per tumor (15 to 20 images from each treatment). Bars represent mean ± SD in each treatment group. (C) 2.5 × 105 4T1 cells were injected into tail veins and indicated treatments were started 7 days later for a total of 11 days. Lungs were harvested and metastases were quantified. N = 5, representative of two separate experiments. *P < 0.05; **P < 0.01 (Mann-Whitney U-test).

Mentions:
The 4T1 tumors were collected at the endpoint and examined for proliferation, apoptosis and vessel density (Figure 6A). Quantification of P-Histone H3, cleaved Caspase-3 and CD31 revealed a significant decrease in cell proliferation and an increase in cell death, which was most prominent and significant in the dovitinib + AEE788-treated group (Figure 6B). The tumor vasculature area and morphology were significantly altered only after combination treatment, primarily as a result of the action of dovitinib as described earlier (Figure 2C). In the experimental metastasis model, treatment with AEE788 alone did not significantly lower the number of lung metastases, while the combination of dovitinib + AEE788 caused a highly significant decrease. These results support the hypothesis that combination therapy effectively inhibits primary tumor outgrowth by impairing proliferation and cell survival; while lung metastases are also very sensitive to blockade of both receptors.

Bottom Line:
The results show that in vivo these breast cancer models become dependent upon co-activation of FGFR and ErbB receptors for PI3K pathway activity.The work presented here shows that in the breast cancer models examined, the combination of dovitinib + NVP-BEZ235 or dovitinib + AEE788 results in strong inhibition of tumor growth and a block in metastatic spread.The resultant decrease in mitosis and increase in apoptosis was consistently stronger in the dovitinib + AEE788 treatment-group, suggesting that targeting ErbB receptors has broader downstream effects compared to targeting only PI3K/mTOR.

Introduction: Targeting receptor tyrosine kinases (RTKs) with kinase inhibitors is a clinically validated anti-cancer approach. However, blocking one signaling pathway is often not sufficient to cause tumor regression and the effectiveness of individual inhibitors is often short-lived. As alterations in fibroblast growth factor receptor (FGFR) activity have been implicated in breast cancer, we examined in breast cancer models with autocrine FGFR activity the impact of targeting FGFRs in vivo with a selective kinase inhibitor in combination with an inhibitor of PI3K/mTOR or with a pan-ErbB inhibitor.

Methods: Using 4T1 or 67NR models of basal-like breast cancer, tumor growth was measured in mice treated with an FGFR inhibitor (dovitinib/TKI258), a PI3K/mTOR inhibitor (NVP-BEZ235) or a pan-ErbB inhibitor (AEE788) individually or in combination. To uncover mechanisms underlying inhibitor action, signaling pathway activity was examined in tumor lysates and transcriptome analysis carried out to identify pathways upregulated by FGFR inhibition. Anti-phosphotyrosine receptor antibody arrays (P-Tyr RTK) were also used to screen 4T1 tumors.

Results: The combination of dovitinib + NVP-BEZ235 causes tumor stasis and strong down-regulation of the FRS2/Erk and PI3K/Akt/mTOR signaling pathways. P-Tyr RTK arrays identified high levels of P-EGFR and P-ErbB2 in 4T1 tumors. Testing AEE788 in the tumor models revealed that the combination of dovitinib + AEE788 resulted in blockade of the PI3K/Akt/mTOR pathway, prolonged tumor stasis and in the 4T1 model, a significant decrease in lung metastasis. The results show that in vivo these breast cancer models become dependent upon co-activation of FGFR and ErbB receptors for PI3K pathway activity.

Conclusions: The work presented here shows that in the breast cancer models examined, the combination of dovitinib + NVP-BEZ235 or dovitinib + AEE788 results in strong inhibition of tumor growth and a block in metastatic spread. Only these combinations strongly down-regulate the FGFR/FRS2/Erk and PI3K/Akt/mTOR signaling pathways. The resultant decrease in mitosis and increase in apoptosis was consistently stronger in the dovitinib + AEE788 treatment-group, suggesting that targeting ErbB receptors has broader downstream effects compared to targeting only PI3K/mTOR. Considering that sub-classes of human breast tumors co-express ErbB receptors and FGFRs, these results have implications for targeted therapy.